Delayed turn-on solid state relay

ABSTRACT

A solid state relay is described which exhibits gradual turn-on characteristics. This relay is useful, for example, for turning ON and OFF traffic control signal lights, wherein the gradual turn-on characteristics of the relay significantly reduce the current surge normally associated with the turn-on of incandescent lamps.

United States Patent [72] Inventor Carlile R. Stevens 1000 Ironwood Place, Alamo, Calif. 94507 [2]] Appl. No. 680,176 [22] Filed Nov. 2, 1967 [45] Patented Oct. 12, 1971 [54] DELAYED TURN-0N SOLID STATE RELAY 10 Claims, 4 Drawing Figs.

[52] U.S.Cl 315/200, 315/205 [51] Int. Cl H05b 37/00 [50] Field of Search 315/200.l, 100 D, 205, 251, 252; 323/22 SC, 24, 38; 307/305, 305 A, 310, 252, 284, 321, 323

[56] References Cited UNITED STATES PATENTS 3,404,328 10/1968 Plow 323/22 X 3,453,063 7/1969 Lewis 323/24 X 3,458,796 7/1969 Cassady 323/22 X 3,018,383 l/1962 Ellert... 307/305 3,129,341 4/1964 Rockafellow... 315/251 X 3,299,344 l/1967 Werts 323/22 Primary Examiner-Roy Lake Assistant ExaminerLawrence J. Dahl Attorney-Roger A. Marrs DELAYED TURN-ON SOLID STATE RELAY BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a gradual turn-on solid state relay, and more particularly to a relay employing silicon-controlled rectifiers which is capable of turning ON an AC current at a gradual rate when applied to the filament of an incandescent lamp.

2. Description of the Prior Art In many applications it is desirable to employ a relay to switch ON and OFF a large current under the control of a signal of lesser magnitude. Mechanical relays suffer the disad vantage of mechanical wear, which limits their useful lifetime. Further, when a mechanical relay is used to control an incandescent bulb, the relay in no way limits the initial current surge applied to the filament when such bulb is turned ON. Such current surges are a prime factor in limiting the lifetime of an incandescent bulb, particularly in applications such as traffic control signal lights where the bulb is turned ON and OFF many times daily.

The application of silicon-controlled rectifiers (SCRs) to the control of AC current is well known. Typically, for example, two such SCRs may be parallel connected in oppositely poled relationship in series with one side of an AC line, and may be used to switch current in that line in response to a control voltage applied to the gates of both SCRs. Solid state relays utilizing SCRs offer the advantage over mechanical relays of not having a limited lifetime.

The current through an SCR may be controlled by varying the point in the AC cycle at which the control voltage is applied to the gate of the SCR. This technique has been applied, for example, to circuits for controlling the brightness of incandescent light bulbs. In such a device, if the SCR is turned on near the end of the cycle, the average current through the lamp filament is small, and the bulb will be dimly lit; if the gate voltage is turned ON near the beginning of the cycle, the average current will be large, and the bulb will be bright.

The present invention provides a solid state relay utilizing simple circuitry which may be activated by an AC or DC control signal. The relay turns ON sufi'rciently gradually so as to substantially reduce the surge of current normally associated with the turn-on of incandescent light bulbs, but sufficiently rapidly that the controlled bulb appears to an observer to go ON abruptly. No external circuitry, such as normally is associated with SCR brightness control devices, is required to accomplish the gradual turn-on.

SUMMARY OF THE INVENTION The inventive gradual turn-on solid state relay utilizes a pair of SCRs parallel-connected in oppositely poled relationship in series with one side of the AC line to be switched. To turn ON the relay, a control voltage is applied to the gate of only one of the SCRs. The voltage drop across the SCR to which the input signal is applied is sensed by a slave circuit which in turn controls the conduction cycle of the second SCR. When the first SCR is turned ON, the slave circuit initially causes the second SCR to turn ON near the end of its cycle; gradually, the second SCR is turned N closer and closer to the start of its cycle. The rate of turn-on may be controlled by appropriate selection of circuit parameters.

Therefore, it is a primary object of the present invention to provide a novel solid state relay for gradually turning ON an AC current.

Another object of the present invention is to provide a gradual turn-on relay employing silicon-controlled rectifiers.

It is another object of this invention to provide a solid state relay which, when used to control an incandescent bulb, limits the initial current surge normally associated with the turn-on of such a bulb.

It is a further object of this invention to provide a gradual turn-on solid state relay wherein one of two SCRs is turned on at a gradually changing point in its conduction cycle, upon sensing that the other SCR has been turned ON by an input control signal.

Yet another object of this invention is to provide a gradual turn-on solid state relay useful for controlling current to a traffic control signal light.

The features of the present invention which are believed to be novel are set forth with particularity in the appended claims. The present invention, both as to its organization and manner of operation, together with further objects and advantages thereof, may best be understood by reference to the following description, taken in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a front elevational view of a typical traffic control signal lightpost incorporating a control box for housing the circuitry of the present invention;

FIG. 2 is a simplified block diagram of a typical traffic light control system employing the inventive gradual turn-on solid state relay;

FIG. 3 is a schematic diagram of a preferred embodiment of the inventive gradual turn-on solid state relay as employed in the control system of FIG. 2; and

FIG. 4 is a schematic diagram of another embodiment of the present invention incorporating a special form of rectifier.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, there is shown a typical traffic control lightpost 10 including red, amber, and green signal lights indicated by numerals 12, 14 and 16, respectively. Control circuitry to activate the various signal lights may be mounted in a housing 18 integrally included in lightpole 10. Typical control circuitry may include a relatively simple time-controlled switch to sequentially turn ON the green, amber, and red signal lights. Alternately, quite complex control circuitry may be used to vary the signal light duty cycle in response to traffic sensing switches, or to maintain a timed relationship to traffic lights at adjacent intersections. Typically, light control circuitry also will have the capability of causing one or more signal lights to flash periodically, a mode of operation often used late at night at intersections experiencing relatively little traffic.

Regardless of the sophistication of the control circuitry, the AC current through each of the red, amber or green signal lights, 12, 14 and 16, typically is controlled by a relay associated with each lamp. As noted hereinabove, typical mechanical or solid state relays presently used to control traffic signal lights do not limit the initial surge of current associated with the turn-on of incandescent lamps. As these current surges adversely affect the lifetime of such lamps, the bulbs used in a typical traffic control signal lightpost must be replaced relatively often. The lifetime of these bulbs is particularly limited at intersections where the signals operate in the flashing mode for many hours daily.

The inventive gradual turn-on solid state relay functions as direct replacement for existing mechanical or solid state relays typically employed with traffic control signal lights. However, the inventive relay provides a gradual turn-on of the current through the controlled lamps, thus significantly reducing the initial current surge, and hence significantly increasing the typical lifetime of the incandescent lamps employed.

FIG. 2 shows a typical traffic control installation using several of the inventive solid state relays 24a, 24b, 24c, 24d, 24e, 24f. In particular, traffic light control circuitry 22, well known to those skilled in the art, provides control signals to turn ON the various red, green and amber lights 12, 14 and 16 in the proper sequence. For example, in the simplified system shown in FIG. 2, control circuitry 22 may first send control signals to relays 24a and 24f to turn ON the East red light 12E and the green light 16N. Subsequently, control circuitry 22 may turn OFF the East green light 12E and turn ON the East amber light 14E.

In each instance, the control signal from traffic light control circuitry 22 typically will be at a relatively low currentsignal, and may be either AC or DC. The control signals activate the appropriate gradual turn-on solid state relays 24a-24f, which in turn switch AC power at a higher current level to the various lights l2, l4 and 16. Using the inventive gradual turn-on solid state relays 240-24), the various signal lights l2, l4 and 16 appear to an observer to go ON abruptly, but in actuality, the current through each of the lights will be turned on gradually so as to reduce the deleterious effects of abrupt turn-on current surges applied to the bulb filaments.

The operation of the gradual turn-on solid state relay may be understood with reference to FIG. 3, which is a schematic diagram of a preferred embodiment of the invention. The relay is designed to be placed in series with one side of the AC line to be controlled; relay terminals 50 and 52 serve as the equivalent of the SPST terminals of a mechanical relay. Either tenninal 50 or terminal 52 may be connected to the AC source; the other terminal then may be connected to the load.

The embodiment of the gradual turn-on relay shown in FIG. 3 includes silicon-controlled rectifiers (SCRs) 30 and 40. Associated with SCR 30 is an input circuit comprising resistors 31, 33 and 34, diode 32 and capacitor 35. The input circuit receives the control signal (for example, from control circuitry 22 of FIG. 2) in response to which the relay is turned ON. Associated with SCR 40 is a slave circuit which accomplishes the gradual turn-on of the relay. This slave circuit comprises diodes 42, 46 and 48, capacitors 43 and 45, and resistors 41 44 and 47.

With terminal 50 connected to an AC source and terminal 52 connected to a load, and with no input signal applied to either of input terminals 54 and 56, on alternate AC half cycles terminal 52 will be positive and negative with respect to terminal 50. (In the following discussion, unless otherwise noted, the polarity of all voltages will be given with respect to reference terminal 50.)

When terminal 52 is negative, SCR 40 will not conduct since it is backward biased, and SCR 30 will not conduct since there is no input voltage applied to its gate. Under these conditions, the entire AC voltage appears across the anode and cathode of SCR 40. Current then flows through diode 48 and resistor 47, charging capacitor 45. When capacitor 45 is charged, the voltage at the junction between capacitor 45 and resistor 47 will be negative.

As the voltage at terminal 52 decreases in negative value, current begins to flow in the path including capacitor 45, diode 46 and capacitor 43. Thus, in effect, part of the energy stored by capacitor 45 is used to charge capacitor 43 to a voltage which, at the junction of diode 42 and capacitor 43, is negative with respect to terminal 52. In a preferred embodiment, the value of capacitor 45 is much larger than the value of capacitor 43, so that capacitor 45 dies not completely discharge during this operation.

When terminal 52 starts to go positive (during the alternate half of the cycle) current does not flow in the path including resistor 41, diode 42 and capacitor 43 until a voltage more positive than that with which the capacitor is charged appears at 52. If there were no charge on capacitor 43, the voltage at the gate of SCR 40 soon would be sufficiently positive to turn ON SCR 40. However, since there is a charge on capacitor 43, the anode of diode 42 never reaches a sufficiently positive voltage to initiate forward conduction of diode 42 and hence, SCR 40. That is, the voltage at terminal 52 never gets sufficiently positive to overcame the negative voltage to which capacitor 43 has been charged. Thus, in the absence of an input signal, the relay remains OFF.

An AC control signal having the same phase as that present at terminal 50 may be employed to turn the relay ON. Such an AC control signal may be connected either to input terminal 54 or, preferably, to terminal 56. With the AC control signal connected to terminal 56, capacitor 35 isolates the gate of SCR 30 from any DC level which may be present at input terminal 56, and eliminates problems (such as a low-impedance return path to the AC source) which may arise when the inventive relay is used in conjunction with other mechanical relays controlled by the same AC control signal. Diode 32 protects the gate of SCR 30 from damage during the half cycle when the AC control signal is negative. Altemately, a lowlevel DC signal may be used to activate the relay; such a signal may be connected to input terminal 54.

Application of an appropriate control signal to input terminal 54 or 56 will cause the relay to go ON. If an AC control signal is used, when terminal 52 goes negative, current will flow through capacitor 35, diode 32 and resistor 31, appropriately biasing gate of SCR 30 to turn it ON. Similarly, a DC input signal will cause current to flow through resistor 34, diode 32 and resistor 31, again biasing ON SCR 30. In either case, SCR 30 conducts during the entire half cycle when terminal 52 is negative.

During the first cycle that a control signal is applied to the relay, capacitor 45 will not be charged, since the voltage drop across SCR 30 is very small. However, by appropriate selection of the values of capacitors 45 and 43, and resistor 44, capacitor 45 will have a residual voltage across it as a result of energy stored when no control signal was applied to the relay. This residual voltage will discharge through capacitor 43, leaving capacitor 43 charged to a voltage which is slightly less than that to which. capacitor 43 became charged when the relay was OFF.

When the voltage at terminal 52 now starts to go positive, current will not flow through capacitor 43, diode 42 and resistor 41 until the voltage at 52 nears its highest value. SCR 40 will reach the point at which conduction may begin at this time; this will occur near the end of the AC half cycle during which terminal 52 is positive. SCR 40 thus will conduct for a small portion of the first AC cycle after the control signal is applied to turn the relay ON.

On the next half cycle (with the control signal still present, and with terminal 52 negative), SCR 30 will conduct and capacitor 45 again will not be recharged. Capacitor 45, which preferably is selected to have a large value compared with that of capacitor 43, still will have a residual charge present, although the charge will be somewhat less than that present during the first AC cycle after the input signal was applied. During this second cycle, capacitor 43 will again be charged (from capacitor 45) but this time to a lesser voltage than during the first cycle. Thus, when the voltage at terminal 52 goes positive, the voltage at the gate of SCR 40 will reach the conduction tum-on point sooner than on the previous cycle, and SCR 40 will conduct for a slightly longer period.

This operation will continue for a number of AC cycles, with SCR 40 gradually increasing its conduction duty cycle. Eventually, the charge on capacitor 45 will be completely dissipated. During succeeding AC cycles, when terminal 52 starts to go positive, SCR 40 will turn ON almost immediately. In effect, the relay then will be fully ON, and will remain ON until the control signal terminates.

It is the gradual change in conduction cycle of SCR 40 at the beginning of the turn-on operation that gives the inventive relay its gradual turn-on capability. The rate at which the relay reaches the full ON condition may be varied by appropriate selection of the values of capacitors 45 and 43, and resistor 44.

Another embodiment of the present invention capable of accomplishing the same slow turn-on characteristic in switching a load is by the use of a Triac 60, as shown in FlG. 4. The Triac is connected in series with the lamp and the power source so that when the conduction occurs, the lamp is illuminated. The Triac is a special form of SCR which conducts in both directions upon the application of either polarity of gate signal. Two methods of activating the Triac as a function of an applied input voltage and time are shown; one is indicated in solid lines and the other in broken lines. In the broken line form, power is fed through a thermistor 61 indicated by the tau symbol directly to the gate of the Triac. When the power is first applied, the resistance of the thermistor is high, therefore insufficient gate power is supplied to the Triac to trigger it until this anode voltage has reached a maximum. As succeeding cycles progress, the currentthrough the thermistor causes it to become warm and in so doing, lowers its resistance, With the lower resistance, more current may flow to the gate of the Triac earlier in the AC cycle causing it to switch sooner. The proper selection of the thermistor curve and Triac characteristic will cause the Triac to commence conduction early in the cycle after appropriate number of cycles have elapsed thus affording a slow tum-on.

[t is often, however, difficult to obtain a Triac which has a predictable gate characteristic so the alternate circuit shown in solid lines is a more reliable implementation. Here, input power flows through a thermistor 62 charging capacitor 63 until breakover voltage has been reached of the breakover diode 64. At this point, the diode suddenly conducts connecting capacitor 63 to the gate of the Triac and utilizing the power stored in capacitor 63 to turn on the Triac at that moment. The resistance of the input thermistor and the value of capacitor 63 may be so chosen that the Triac will not turn ON until near the end of the AC cycle, when power is first applied. However, as the thermistor warms up and its resistance lowers, the voltage rises faster and faster on capacitor 63 breaking over diode 64 until breakover is accomplished at the beginning of the AC cycle causing the lamp to be fully illuminated.

In addition to the circuit of FIG. 3 and also applied to the circuit of FIG. 4, there is a neon indicating lamp 65 and 66, respectively, and a series dropping resistor 67 and 68, respectively, to control the current through the lamp. This lamp is connected directly across the entire relay and is illuminated when the relay is not operating. This gives the user of traffic control equipment the advantage of a visual indication of the relays in the control box rather then to look directly at the lights themselves.

While the relays have been described in conjunction with an application to traffic control signal lights, the invention is by no means so limited, and the gradual turn-on solid state relay may be used in any other relay application wherein a gradual increase in the controlled current is desirable.

While a particular embodiment of the present invention has been shown and described, it will be obvious to those skilled in the art that changes and modification may be made without departing from this invention in its broader aspects, and, therefore, the aim in the appended claims is to cover all such changes and modifications as fall within the true spirit and scope of this invention.

lclaim:

l. A solid state relay comprising:

first and second silicon-controlled rectifiers connected in back-to-back parallel, oppositely poled relationship;

said second silicon-controlled rectifier connected across said first silicon-controlled rectifier so as to be operated thereby; means for providing a gate voltage to turn ON said first silicon-controlled rectifier in response to a control signal;

means responsive to the voltage drop across said first silicon-controlled rectifier for controlling a conduction cycle of said second silicon-controlled rectifier;

said conduction cycle being defined as the initiation of operation of said second silicon-controlled rectifier at a delayed time; and

means for connecting said first and second silicon-controlled rectifiers in series with a load and an AC power source.

2. A relay as defined in claim 1 wherein said last-mentioned controlling means comprises:

a first capacitor charged by said voltage drop; and

means cooperating with said first capacitor to control the gate voltage of said silicon-controlled rectifier in response to the charge on said first capacitor.

3. A slave circuit for controlling the conduction cycle of a second silicon-controlled rectifier in response to the voltage drop across a first silicon-controlled rectifier, said first and second silicon-controlled rectifiers being reversely parallelconnected, said circuit comprising:

a first capacitor, a first resistor and a first diode connected in series across the anode and cathode of said second rectifier;

a second diode, the cathode of which is connected to the junction of said first resistor and said first diode;

a second resistor and a second capacitor connected in parallel between the anode of said second diode and the anode of said second rectifier;

a third resistor and a third diode connected in series between the junction of said second diode and said second resistor and said second capacitor and the cathode of said second rectifier; and

the gate of said second rectifier being connected to the junction of said third resistor and said third diode.

4. The invention as defined in claim 2 including a neon indicating lamp and a series dropping resistor connected directly across said rectifiers and being adapted to visually indicate when said rectifiers are not operating properly.

5. A solid state relay for applying an AC current at a gradual rate to the filament of an incandescent lamp comprising:

a silicon-controlled rectifier characterized by its ability to conduct in both directions upon the application of either polarity of a gate signal; and

a thermistor directly coupled to said rectifier for activation thereof as a function of the applied AC voltage.

6. The invention as defined in claim 5 including a neon in dicating lamp and a series dropping resistor connected directly across said rectifier adapted to indicate when the relay is not operating properly.

7. A solid state relay for applying an AC current at a gradual rate to the filament of an incandescent lamp comprising:

a silicon-controlled rectifier characterized by its ability to conduct in both directions upon the application of either polarity of a gate signal;

a thermistor characterized to operate as a function of the applied AC voltage thereto;

a capacitor coupled to said thermistor and being chargeable in response to operation of said thermistor to develop a breakover voltage; and

a diode directly coupled between said thermistor and said rectifier operable in response to the developed breakover voltage to apply the stored charge in said capacitor at said gate signal to activate said rectifier.

8. In a solid state relay of the type utilizing first and second parallel-connected, reversely poled silicon-controlled rectifiers, the improvement comprising:

means responsive to a voltage drop across said first silicon controlled rectifier, for controlling the conduction cycle of said second silicon-controlled rectifier;

a first capacitor charged by said voltage drop;

means cooperating with said first capacitor to control a gate voltage of said second silicon-controlled rectifier in response to the charge on said first capacitor;

a second capacitor; and

means for charging said second capacitor to a reference voltage responsive to the charge in said first capacitor, and wherein said voltage is proportional to the algebraic sum of said reference voltage and the voltage at the anode of said silicon-controlled rectifier.

9. In a solid state relay of the type utilizing a first and a second parallel-connected, reversely poled silicon-controlled rectifiers, the improvement which comprises:

said second silicon-controlled rectifier electrically connected across said first silicon-controlled rectifier;

circuit means responsive to the absence of a voltage drop across said first silicon-controlled rectifier and operable for controlling conduction of said second silicon-controlled rectifier;

said conduction being defined as the initiation of operation of said second silicon-controlled rectifier at a delayed tlme;

means for connecting said first silicon-controlled rectifier 10. A relay as defined in claim 9 wherein said means for and said second silicon-controlled rectifier in series with a controlling comprises: load and an AC power source; and wherein a first capacitor charged by said voltage drop; and

said delayed time is defined as meaning conduction of said means cooperatiifg with Said ff p 10 Comm] a 8 fi t ili t fl d tifi within a fi t AC cycle 5 voltage of said second silicon-controlled rectifier in while conduction of said second silicon-controlled rectifiresponse to the charge first Capacltor' er occurs several AC cycles later. 

1. A solid state relay comprising: first and second silicon-controlled rectifiers connected in back-to-back parallel, oppositely poled relationship; said second silicon-controlled rectifier connected across said first silicon-controlled rectifier so as to be operated thereby; means for providing a gate voltage to turn ON said first silicon-controlled rectifier in response to a control signal; means responsive to the voltage drop across said first siliconcontrolled rectifier for controlling a conduction cycle of said second silicon-controlled rectifier; said conduction cycle being defined as the initiation of operation of said second silicon-controlled rectifier at a delayed time; and means for connecting said first and second silicon-controlled rectifiers in series with a load and an AC power source.
 2. A relay as defined in claim 1 wherein said last-mentioned controlling means comprises: a first capacitor charged by said voltage drop; and means cooperating with said first capacitor to control the gate voltage of said silicon-controlled rectifier in response to the charge on said first capacitor.
 3. A slave circuit for controlling the conduction cycle of a second silicon-controlled rectifier in response to the voltage drop across a first silicon-controlled rectifier, said first and second silicon-controlled rectifiers being reversely parallel-connected, said circuit comprising: a first capacitor, a first resistor and a first diode connected in series across the anode and cathode of said second rectifier; a second diode, the cathode of which is connected to the junction of said first resistor and said first diode; a second resistor and a second capacitor connected in parallel between the anode of said second diode and the anode of said second rectifier; a third resistor and a third diode connected in series between the junction of said second diode and said second resistor and said second capacitor and the cathode of said second rectifier; and the gate of said second rectifier being connected to the junction of said third resistor and said third diode.
 4. The invention as defined in claim 2 including a neon indicating lamp and a series dropping resistor connected directly across said rectifiers and being adapted to visually indicate when said rectifiers are not operating properly.
 5. A solid state relay for applying an AC current at a gradual rate to the filament of an incandescent lamp comprising: a silicon-controlled rectifier characterized by its ability to conduct in both directions upon the application of either polarity of a gate signal; and a thermistor directly coupled to said rectifier for activation thereof as a function of the applied AC voltage.
 6. The invention as defined in claim 5 including a neon indicating lamp and a series dropping resistor connected directly across said rectifier adapted to indicate when the relay is not operating properly.
 7. A solid state relay for applying an AC current at a gradual rate to the filament of an incandescent lamp comprising: a silicon-controlled rectifier characterized by its ability to conduct in both directions upon the application of either polarity of a gate signal; a thermistor characterized to operate as a function of the applied AC voltage thereto; a capacitor coupled to said thermistor and being chargeable in response to operation of said thermistor to develop a breakover voltage; and a diode directly coupled bEtween said thermistor and said rectifier operable in response to the developed breakover voltage to apply the stored charge in said capacitor at said gate signal to activate said rectifier.
 8. In a solid state relay of the type utilizing first and second parallel-connected, reversely poled silicon-controlled rectifiers, the improvement comprising: means responsive to a voltage drop across said first silicon-controlled rectifier, for controlling the conduction cycle of said second silicon-controlled rectifier; a first capacitor charged by said voltage drop; means cooperating with said first capacitor to control a gate voltage of said second silicon-controlled rectifier in response to the charge on said first capacitor; a second capacitor; and means for charging said second capacitor to a reference voltage responsive to the charge in said first capacitor, and wherein said voltage is proportional to the algebraic sum of said reference voltage and the voltage at the anode of said silicon-controlled rectifier.
 9. In a solid state relay of the type utilizing a first and a second parallel-connected, reversely poled silicon-controlled rectifiers, the improvement which comprises: said second silicon-controlled rectifier electrically connected across said first silicon-controlled rectifier; circuit means responsive to the absence of a voltage drop across said first silicon-controlled rectifier and operable for controlling conduction of said second silicon-controlled rectifier; said conduction being defined as the initiation of operation of said second silicon-controlled rectifier at a delayed time; means for connecting said first silicon-controlled rectifier and said second silicon-controlled rectifier in series with a load and an AC power source; and wherein said delayed time is defined as meaning conduction of said first silicon-controlled rectifier within a first AC cycle while conduction of said second silicon-controlled rectifier occurs several AC cycles later.
 10. A relay as defined in claim 9 wherein said means for controlling comprises: a first capacitor charged by said voltage drop; and means cooperating with said first capacitor to control a gate voltage of said second silicon-controlled rectifier in response to the charge on said first capacitor. 